Field
The disclosed concept pertains generally to transfer switches, and, more particularly, to an automatic transfer switch including a load management system. The disclosed concept also pertains to load management methods for automatic transfer switches.
Background Information
Transfer switches are employed to protect critical electrical loads against loss of power from a primary power source (e.g., utility power) backed up by a secondary power source (e.g., a generator). The transfer switch is electrically connected to both of the power sources and supplies a number of loads with power from one of the two power sources. In the event that power is lost from the primary power source, the transfer switch transfers the load to the secondary power source. This transfer can be manual or automatic. After the primary power source is restored, the load is transferred back to the primary power source.
In an automatic transfer switch (ATS), a controller or other intelligence of the ATS initiates the transfer when the primary power source fails or falls below, or rises above, a predetermined voltage and/or frequency. If the secondary power source is a standby generator, then the ATS initiates generator startup and transfers to the secondary power source when sufficient generator voltage is available. When the primary power source is restored, the ATS automatically transfers the load back to the primary power source and initiates generator shutdown. In the event that the primary power source fails and the secondary power source does not appear, then the ATS remains electrically connected to the primary power source until the secondary power source does appear. Conversely, if electrically connected to the secondary power source and the secondary power source fails while the primary power source is still unavailable, then the ATS remains electrically connected to the secondary power source.
An ATS automatically performs the transfer functions and can include, for example and without limitation, a power contactor or two circuit breakers to implement the ATS operations, and a controller (or other intelligence or supervisory circuit) to constantly monitor the condition of the power sources and provide automatic control of the switch and related circuit operation.
Certain systems and installations (e.g., without limitation, data centers; hospitals; water treatment plants; other critical processes that need the constant supply of power to a system load) employ a power system structured to provide an uninterruptable power supply (UPS). UPSs are well known. The primary power source is usually an electric utility or the public power grid, and the secondary power source is usually a generator. Alternatively, there could be two generators used instead of the utility or public power grid, or there could be two utilities. Basically, any suitable power sources that need to be switched with a load output can be employed.
In many situations, the secondary power source has a limited capacity and may not be capable of providing power to all the loads. For example, an air conditioning system draws a significant amount of power and when the secondary power source is a backup generator with a limited capacity, it may not be able to provide enough power to the air conditioning system. One prior transfer switch is designed for use with air conditioning systems and a generator. The transfer switch monitors a thermostat input to start the air conditioning. When the transfer switch receives the thermostat input, it then checks the percentage of maximum generator output that the generator is operating at. If the generator output is below 80% of the maximum generator output, the transfer switch allows the air conditioning system to start. If the generator output is at 80% or higher of the maximum generator output, the transfer switch does not allow the air conditioning system to start.
While the prior transfer switch aims to prevent the connected generator from becoming overburdened by the air conditioning system, it sacrifices efficiency of use of the generator and still presents a risk of allowing the generator to become overburdened. For example, the air conditioning system may draw less than 20% of the maximum output of the generator, and thus the prior transfer switch may prevent operation of the air conditioning system even though it would not overburden the generator. In another case, the air conditioning system may draw greater than 20% of the maximum output of the generator, in which case the transfer switch would allow the air conditioning system to operate even though it would overburden the generator. With limitless combinations of air conditioning systems and generators, the amount of the maximum output of the generator the air conditioning system uses may vary considerably. Thus, the prior transfer switch cannot provide efficient use of the generator as well as protecting it from becoming overburdened.
There is therefore room for improvement in transfer switches.